I'm not an expert on this, but this is what has worked for me. This setup (or very similar) should work with other Windows Mobile v5 devices.
1. Install The Windows Mobile Modem Scripts Onto Your Mac
Download the Generic Windows Mobile Modem Scripts (3 of them) from: http://www.taniwha.org.uk/
The scripts are under "Windows Smartphones & BlackBerrys" called "Scripts for Windows Mobile phones" (half way down the page)
Once downloaded, put the scripts into >Library >Modem Scripts
2. Start Bluetooth on the Treo
Go >Start Menu >Settings >Connections (from the bottom) >Bluetooth
Tick "Turn on Bluetooth"
Tick "Make this device discoverable to other devices"
3. Pair The Treo with your Mac (done on the Mac)
If the Bluetooth menu is not already on your mac (near the date and time, top right) go to >System Preferences >Bluetooth
Under the "settings" tab tick "Show Bluetooth status in the menu bar" and the menu will appear
Once the menu is there, choose "Set up Bluetooth Device..."
Click "Continue" and on the next screen choose "Mobile Phone". Click "Continue"
Make sure the screen on the Treo is on (I've found the mac has trouble discovering the device otherwise)
Choose your phone and click "Continue"
Click continue and enter the pass code on the Treo and click "Next"
Once pared, on the phone click "Finish"
On the mac, make sure "Access the Internet with your phone's data connection" is ticked and click "Continue"
Enter your Vodafone account username and password (see notes below)
Under phone number enter: *99# (this invokes GPRS or the 3G data connection)
Choose "Windows Mobile GSM 460k" from the "Modem Script" pop up menu
Select the new phone from the Bluetooth Devices list and press the "Edit Serial Ports" button from the left
Click the plus sign under the main window
Name it "Treo750" (or Vodafone or whatever you want)
Choose "Modem" from the "Port type" pop up menu
Tick "Require pairing for security" and "Show in Network Preferences"
4. Configure The Bluetooth Port
Go >System Preferences >Network
A "New Port Detected" window should pop up, click "OK" (if it doesn't and you have a new "Bluetooth" port, don't worry)
Select the "Bluetooth" port from the list and click "Configure"
Under the PPP tab, click the "PPP Options" button from the bottom
Untick "Send PPP echo packets" and "Use TCP header compression" under "Advanced Options"
Click the "Apply Now" button from the bottom right
Close System Preferences
5. Connecting to browse the internet from the mac
Go >Applications >Internet Connect
Click "Bluetooth" from the top
Tick "Show modem status in menu bar"
Quit internet connect
Make sure the Treo is on and bluetooth is on (and your data connection is on, see notes below)
Go up to the internet connect menu (the little phone icon) and select "Connect"
For the carrier account name and password, I just used the same account info that was on my phone under >Start >Settings >Connections >Connections Icon >Manage existing connections >Edit "VFNZ Internet" >Next Screen >Next Screen >Next Screen. On this third screen, I found my account details. If you don't know your account password probably ring Vodafone to find out.
I've found the connection to be a little bit unreliable when the Treo's screen goes to sleep - so I tend to hit a button on the treo every 30 seconds to keep it on.
I had trouble the first time I tried to connect through internet connect. But after I connected through >System Preferences >Network >Bluetooth >Connect button next to the configure button at the bottom, it worked every time from the Internet Connect menu.
Also, you may want to check your email or browse a page on the Treo first, just before connecting from your mac to make sure the data connection is on - I haven't worked this out fully but I'm not sure the Mac can open the data connection itself.